Among various optical instruments for measuring displacement, which can detect variations in position (length of displacement) of a diffraction grating by making diffracted light interfere, using a displacing diffraction grating as a scale, are known a type of instrument for which variations in wavelength are essentially permitted, in which detection of displacement is effected by utilizing interference between diffracted light beams of same order with different signs, plus and minus, as disclosed in the publication of Japanese Utility Model No. 81510/1982 or the provisional publication of Japanese Patent No. 191907/1983, and also another type of instruments in which detection of displacement is effected by utilizing interference between diffracted light beams of 1st order, as disclosed in Japanese Patent Application No. 205956/1983.
Since all of these prior art optical instruments for measuring displacement are so designed that their optical system is not subject to influences, such as elimination of interference, etc., due to variations in wavelength of the light source within a permissible range, they have an advantage that it is possible to use a light source, which is defective in wavelength stability, but is not expensive, such as a semiconductor laser device.
On the other hand, however, in order that these optical systems can manifest their desired characteristics, it is necessary to adjust them in such a manner that optical path lengths of two beams split by means of a beam splitter or diffraction grating in their interferometer, diffracted and finally made interfere vary always equally. The reason why this is absolutely necessary is as follows. If their optical path lengths differed from each other, phase variations in interference signals would be caused by variations in wavelength at the same time as phase variations due to displacement of the diffraction grating which is to be measured, which gives rise to measurement errors.
In order to avoid such a drawback, adjustment precision of the optical path lengths of the two beams mentioned above should be, for example, from several 10 .mu.m to several 100 .mu.m in the case where a semiconductor laser device is used without temperature compensation, although it is not possible to speak generally, because it depends on variations in wavelength which are to be allowed and required precision as well as on the displacement length of the diffraction grating, to which a period of the interference signals corresponds.
It has been thought that for this purpose a high precision supporter for an optical system or a jig for positioning it was necessary. The former was expensive and the latter gave rise to problems in that high precision was required for adjustment, and that when fixing means such as screws were loosened, troubles were often caused in the optical system, which caused measurement errors. Furthermore, in the case where a coherent light source such as a single mode laser and so forth was used, it was necessary to dispose expensive non-reflection coating on optical parts, because it was feared that phase shifts might be produced in interference signals due to superposition of unnecessary reflected light coming from surfaces of optical parts.
In spite of these countermeasures it was difficult to keep adjustment precision within a desired range.